1. Field of the Invention
The present invention relates to an apparatus for treatment of blood and more particularly to an apparatus for removing undesirable substances from blood plasma, and in which only the plasma is separated from a blood stream by a plasma separating membrane which prohibits the penetration of blood cellular components but allows the plasma to pass therethrough. In the apparatus the plasma is, after the removal of the undesirable substances, returned to the blood stream again through said plasma separating membrane.
2. Description of the Prior Art
Three methods of treating blood by the use of a blood treating material have been reported:
1. A direct hemoperfusion method (hereinafter called the "DHP method"); PA1 2. A plasma perfusion method (hereinafter called the "PP method"); and PA1 3. A method of practicing the PP method by using a membrane-type plasma separator filled with a blood treating material (hereinafter called the "CPP method"). An example is described in U.S. Pat. No. 4,013,564, and in Jinko Zoki, Vol. 9, No. 2, pp 506-509.
Of these methods, the DHP method has already been put in practice commercially. However this method, in which the blood is brought into direct contact with the blood treating material, has disadvantages, such as that it causes a decrease in blood platelets and white blood cells in the perfused blood, thrombogenesis, hemolysis, and microparticles of the blood treating material are sometimes released into the blood being treated. One countermeasure to these disadvantges is to coat the blood treating material with a substance which is highly biocompatible with the blood. However, in the present state of the technology, the use of such a coating is inevitably accompanied by a lowering of efficiency and an increase in the complexity of the process of manufacturing the blood treating material.
The PP method in which the blood itself is not brought into direct contact with the blood treating material, is free from such degeneration of the blood cellular components described above, but the apparatus, including the blood circuit, is complicated because this method requires a separate pump in the plasma circuit and a filter through which the treated plasma is returned to the blood stream. In addition, it is a further disadvantage of this method that a larger volume of extracorporeal circulating blood is required.
The CPP method not only is free from the problem of the direct contact between the blood and the blood treating material but also has an advantage that the problem of microparticles of the blood treating material being released into the blood is eliminated because the plasma separating membrane functions as a microfilter. Therefore, consideration has been given to putting this method into practice in two ways using either a hollow fiber membrane or a flat membrane. One way is to remove the undesired solutes by dispersing them through a so-called dialysing membrane 1, as shown schematically in FIG. 1. The other way is to treat the plasma with a blood treating material, as is shown in FIG. 2, causing only the plasma component 5 to separate from a blood stream path 4 through a high-flux membrane 2 which separates blood treating material 3 from the blood stream, flow through the blood treating material 3, and then return to the blood stream. However, the former way, which is really outside the scope of the PP method, is not suitable for treating substances having a molecular weight higher than a medium value, while the latter way can hardly be expected to work effectively, if carried out in the way described, because the plasma permeation rate through the membrane is not constant and stable, but depends on the pressure resistance in the blood stream path 4 and the permeability of the membrane 2. Furthermore, the entire stream of the plasma has such a simple flow pattern that the plasma flows toward the blood treating material 3 in the vicinity of the blood stream entrance and returns to the original blood stream path 4 in the vicinity of the blood stream exit 4, as is indicated by arrows 5 in FIG. 2, so that only about half the area of the membrane 2 carries out filtration in the vicinity of the entrance and the remaining half is used for the return of the treated plasma to the blood stream. Accordingly, in the region of the entrance of the blood stream path 4 there is formed a protein gel layer, which causes a substantial decrease of the effective area of the membrane 2, producing a serious disadvantage that a stable flow of blood is disturbed.